Various optical devices are known which employ prisms upon which a finger whose print is to be identified is placed. The prism has a first surface upon which a finger is placed, a second surface disposed at an acute angle to the first surface through which the fingerprint is viewed and a third illumination surface through which light is directed into the prism. In some cases, the illumination surface is at an acute angle to the first surface, as seen for example, in U.S. Pat. Nos. 5,187,482 and 5,187,748. In other cases, the illumination surface is parallel to the first surface, as seen for example, in U.S. Pat. Nos. 5,109,427 and 5,233,404. Fingerprint identification devices of this nature are generally used to control the building-access or information-access of individuals to buildings, rooms, and devices such as computer terminals.
One of the problems associated with fingerprint sensors concerns the reliable and accurate transformation of the ridge-and-valley pattern of the fingertip into electrical or optical signals to be stored in a digital format. Optical systems as described above, for example using a prism, require sophisticated equipment and tend to be bulky and costly.
In an attempt to overcome some of the limitations and disadvantages of using optical systems based on illumination of the finger tip, U.S. Pat. No. 4,353,056 in the name of Tsikos issued Oct. 5, 1982, discloses an alternative kind of fingerprint sensor that uses a capacitive sensing approach. The described sensor has a two dimensional, row and column, array of capacitors, each comprising a pair of spaced electrodes, carried in a sensing member and covered by an insulating film. The sensors rely upon deformation to the sensing member caused by a finger being placed thereon so as to vary locally the spacing between capacitor electrodes, according to the ridge-and-valley pattern of the fingerprint, and hence, the capacitance of the capacitors. In one arrangement, the capacitors of each column are connected in series with the columns of capacitors connected in parallel and a voltage is applied across the columns. In another arrangement, a voltage is applied to each individual capacitor in the array. Sensing in the respective two arrangements is accomplished by detecting the change of voltage distribution in the series connected capacitors or by measuring the voltage values of the individual capacitances resulting from local deformation. To achieve this, an individual connection is required from the detection circuit to each capacitor.
While the described sensor may not suffer from the problems associated with the kind of sensor employing an optical sensing technique, it suffers from its own problems. For example, applying a voltage to the array of capacitors requires circuitry to each capacitor for charging. Such charging also requires further states in the imaging process consuming more resources and providing added areas for unreliability. Moreover, the need to provide a respective connection to each individual capacitor in the array means that a very large number of connecting lines is necessary. This creates difficulties, both in the fabrication of the sensing member and its interconnection with the detection circuit.
In yet another attempt to improve upon deficiencies and limitations of the aforementioned and other prior art, a further contact imaging device is described in U.S. Pat. No. 5,325,442 in the name of Knapp, issued Jun. 28, 1994. Those parts of the disclosure of this patent not included in this specification are incorporated herein by reference.
Knapp describes making a capacitance measuring imaging device in the form of a single large active matrix array involving deposition and definition by photolithographic processes of a number of layers on a single large insulating substrate. Electrodes and sets of address conductors formed of metal and field effect transistors are formed as amorphous silicon or polycrystalline silicon thin film transistors (TFTs) using an appropriate substrate of, for example, glass or quartz.
A fingerprint sensing device and recognition system that includes an array of closely spaced apart sensing elements each comprising a sensing electrode and an amplifier circuit is described in U.S. Pat. No. 5,778,089 in the name of Borza, issued Jul. 7, 1998. The device is used to sense electrical charge on a fingertip and obviates the need to pre-charge the sensing electrode. The device may be constructed with a single die or with multiple dies. Those parts of the disclosure of this patent not included in this specification are incorporated herein by reference.
Fingerprint analysis is perhaps the most widely used and studied biometric technique. The analysis of fingerprints is discussed in the following references which are hereby incorporated by reference:                Xiao Qinghan and Bian Zhaoqi,: An approach to Fingerprint Identification By Using the Attributes of Feature Lines of Fingerprint,” IEEE Pattern Recognition, pp 663, 1986;        C. B. Shelman, “Fingerprint Classification—Theory and Application,” Proc. 76 Carnahan Conference on Electronic Crime Countermeasures, 1976;        Feri Pernus, Stanko Kovacic, and Ludvik Gyergyek, “Minutiae Based Fingerprint Registration,” IEEE Pattern Recognition, pp 1380, 1980;        J. A. Ratkovic, F. W. Blackwell, and H. H. Bailey, “Concepts for a Next Generation Automated Fingerprint System,” Proc. 78 Carnahan Conference on Electronic Crime Countermeasures, 1978;        K. Millard, “An approach to the Automatic Retrieval of Latent Fingerprints,” Proc. 75 Carnahan Conference on Electronic Crime Countermeasures, 1975;        Wegstein, An Automated Fingerprint Identification System, NBS special publication, U.S. Department of Commerce/National Bureau of Standards, ISSN 0083-1883; no. 500-89, 1982; and        Moenssens, Andre A., Fingerprint Techniques, Chilton Book Co., 1971; and,        Wegstein and J. F. Rafferty, The LX39 Latent Fingerprint Matcher, NBS special publication, U.S. Department of Commerce/National Bureau of Standards; no. 500-36, 1978.        
Typically in electronic fingerprint matching, a live fingerprint is scanned and electronically digitized. The digitized data generally contains information pertaining to characteristic features of the fingerprint, such as ridge endings, points of ridge bifurcation, and the core of a whorl i.e., fingerprint minutiae. The digitized data is then compared with stored data relating to fingerprints that have been obtained previously from corresponding authorized persons i.e., fingerprint templates. When a match is detected, within a predetermined level of security in the form of a predetermined false acceptance rate, the individual is identified and a corresponding action is performed.
In general, there are two types of errors associated with fingerprint identification. The first is false reject (Type I) error and the second is false accept (Type II) error. False accept errors occur when there are enough similarities between fingerprints of two individuals, that one is mistaken for the other. A false reject error occurs for a variety of reasons, and refers to when an individual is not identified even though the individual is an authorized user registered with the system.
A moisture dependent variation of the ridge-and-valley pattern of a same fingertip often leads to an increased incidence of Type I errors, which causes undue inconvenience to users of a fingerprint identification system. For instance, seasonal weather patterns, high levels of physical exertion, the user wearing gloves to warm his hands, etc. can cause a fingertip to be excessively dry or excessively moist relative to a template fingerprint that was provided during an initial enrollment operation. A warm, moist fingertip tends to be swollen slightly, such that adjacent ridges are pushed more closely together and may connect (a ridge inter-connect). Further, the fingerprint identification system may mistakenly register sweat or moisture that is trapped within a valley as a fingertip ridge. In contrast, a cool, dry fingertip tends to be constricted slightly, such that portions thereof fail to make proper contact with the imaging surface, causing the fingerprint identification system to register gaps or discontinuities where there is in fact a continuous ridge. Of course, when a sample image from one of a moist and a dry fingertip is compared to a reference image obtained previously under ideal conditions, there is an increased likelihood that the user will be falsely rejected.
U.S. Pat. No. 6,241,288, issued Jun. 5, 2001 to Bergenek et al., teaches a fingerprint identification/verification method using stored bitmap images of a reference fingerprint to correlate with a bitmap image of an input fingerprint. The method includes a user enrollment step, during which step an imaging device captures a binarized reference image of a fingerprint presented by the user. The binarized reference image is checked for ‘dryness’ or ‘wetness’ as part of a quality control portion of the enrollment step. If the image is ‘too dry’, the pressure applied to the sensor was too light or the sensor failed to detect parts of ridges because of fingertip dryness. If the image is ‘too wet’, moisture on the fingertip ‘flooded’ the fingerprint valleys. Wetness or dryness is detected by analyzing the image for too few dark pixels (dryness) or, too many dark pixels and continuous dark areas (wetness). If the reference image is rejected, the person is asked to correct the problem and another reference image is taken. Optionally, the input fingerprint is subjected to a similar quality control process during a recognition step of the method, for instance when the user wishes to be authenticated. Unfortunately, the onus is on the user to correct the problem, causing the user to suffer inconvenience unnecessarily.
U.S. Pat. No. 6,131,464, issued Oct. 17, 2000 to Pare, Jr. et al., discloses a pressure sensitive biometric input apparatus and a method of operating same. An image of a fingertip under investigation is captured at an imaging area of the apparatus. If the image is too dark, the pressure at which the image was scanned is examined. If the pressure is at the higher end of a predetermined pressure range, a preferred embodiment of the invention instructs the user to use less pressure, postulating that the user might have well-defined ridges and a decrease in pressure might well lighten the image to an acceptable level. Otherwise, an embodiment of the invention concludes that the most likely cause of the problem is too much moisture, and instructs the user to dry his or her finger and retry the scan. Unfortunately, the user must attempt multiple fingerprint scans before an acceptable match is obtained. Further unfortunately, the onus is on the user to correct the problem, causing the user to suffer inconvenience unnecessarily.
It would be advantageous to provide a method and a system to process data indicative of a fingerprint image and to determine a value in dependence upon a moisture level thereof, prior to attempting user authentication. The determined value for selecting a particular reference fingerprint of a plurality of reference fingerprints, each of the plurality of reference fingerprints obtained at a known fingertip moisture state.